1. Field of the Invention
Embodiments of the invention generally relate to lithography, and more particularly to linear motors for a vacuum environment.
2. Background Art
Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device, which is alternatively referred to as a mask or a reticle, is typically used to generate a circuit pattern to be formed on an individual layer in an IC. This pattern is transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned.
Linear motors are used with devices used to hold, for example, a patterning device (e.g., a mask or reticle), a wafer, etc., in order to move the patterning device and/or the wafer in X, Y, or Z directions. The linear XY motors can include flat electrical coils that are aligned within the high flux regions produced by X and Y magnetic circuits. The flat electrical coils are sandwiched with stainless steel water cooling jackets. The outer cooling jackets are brazed and welded to the overall coil housing, forming a seal coil enclosure. An important layer in assembling of the linear motors is a potting compound layer that (1) mechanically connects the coils to the cooling plates and the housing; (2) carries heat from the coils to the cooling plates; and (3) is the compliant layer for the thermal expansion of the coils. These functions are critical for operation of the motor. In particular, a thicker layer of the potting compound would be better for compliancy but bad for thermal conductivity, whereas a thin layer of the potting compound would be beneficial for thermal conductivity, but bad for compliancy.
The thickness of the layer of the potting compound between the coils and the cooling plates is affected by the dimensional variability of all other components of the motor. This layer is important from both a mechanical and a thermal point of view and the range of variability is such that a considerable portion of the manufactured motors would not meet the thermal requirements or would have reliability issues.
Welding of the motor housing introduces additional problems. Welding of the bottom cooling plate to the housing is one of the last manufacturing steps (because of the way the motor is assembled). Welding the bottom cooling plate to the housing as one the last manufacturing steps can result in burning the electrical insulation and therefore, the motor would not be vacuum tight. If the welding fails for any reason, a fully assembled motor would be lost without possibility of recovering any component.
Another draw back of prior motor designs is that the testability of sub-components is very limited. Some critical properties, such as thermal resistance and mechanical bonds strength, can only be measured and tested at the end of the motor manufacturing process.